Control Mount with arrow keys

This MR adds a feature that allows the user to control the mount via arrow keys (feature request).

Corresponding keys are highlighted when pressed. It's possible to move ra and dec at the same time though 2 keys will be highlighted instead of their diagonal button counterpart. A de-flicker function is added to mitigate key flickering problem on certain device / OS (ubuntu 18.04 seemed to have this issue as qt 5.9 had an issue detecting autorepeating key)